Railroads are typically constructed to include a pair of elongated, substantially parallel rails, which are coupled to a plurality of laterally extending ties. The ties are disposed on a ballast bed of hard particulate material such as granite. Over time, normal operations on the railroad may cause the rails to deviate from a desired geometric orientation.
Rail maintenance processes for addressing such concerns typically involve the use of a tamping machine with a versine based measurement system composed of a system of buggies and chord measurement devices, which cooperate with each other to provide a reference system to measure the position of the track prior to applying the desired corrections to the track. A typical correction process involves lifting rail with mechanical clamps, aligning the track by shifting it to a calculated lateral position, and then tamping the ballast under each tie to hold the track in the desired position. This work sequence is typically repeated at each tie during the course of the correction process.
Reference points are used to establish a geometry of the track at the particular location being worked. That is, the recorded values are used to triangulate the geometry of the section of track being worked, while an onboard computer compares the previous section of track already corrected to the current section and makes the calculations for the required corrections to be made at the work heads.
In the railway industry, track geometry measurement is used to measure the spatial relationship of one rail with reference to another. This can be achieved using a chord based measurement by hand, or an automated chord based measurement system using a contact measuring device with respect to a rigid frame. The resulting data from these measurement systems is used to specify various maintenance activities, such as tamping.
The precision and accuracy of the track geometry measurement requirements vary based on operations. In the case of a high speed line on which trains travel at a high speed (for example over 200 kph), an acceptable wavelength for track deviations can be quite high. For example, to damp oscillations and limit suspension movement at a frequency of 1 Hz, a distance of a wavelength from a peak through a valley to a next peak may be 200 m or greater. For slower speed lines (<100 kph), wavelengths of 20 m are considered,
When considering tamping activities, it is also necessary to calculate a correction to the track geometry, based on either a smoothing of the measured track, or with reference to a defined location in space. During tamping activities the track position may be changed in the area of only some millimeters up to several centimeters. Thus, very precise measurements over long distances may be needed.
For some of these corrections (tamping to an absolute track position and not only smoothing of the track geometry) additional measurements are carried out to acquire the absolute position of the track relative to track-side reference points considered to be fixed in space. Such reference points are often mounted on catenary masts, other fixed objects, survey markers, etc.
To measure absolute position of the track at discrete locations, the position of the track may be measured relative to reference points by manual or semi-manual measurement using hand laser tools and D-GPS. However, measurements using these methods are time-intensive (hand laser tools) and relatively inaccurate (D-GPS—when used for measurements under a normally used period of time).
Measurements carried out with laser measurement systems to acquire the position of the track relative to the track-side reference points may be used for tamping operations. However, these laser measurement systems require a first operator team in front of the vehicle to place measurement equipment on the track rails to measure the position of the track. A second operator team is required behind the vehicle to place measurement equipment on the track rails after the vehicle has performed work to verify the adjusted position of the track. The presence of the operator team working on the track also leads to safety personnel being required to secure the work of the measurement team. In sum, 2-6 persons per tamping shift may be required to perform these measurements. Thus, laser measurement systems are slow and labor intensive. Further, laser measurement generally requires some kind of operator interaction to carry out.
To obtain accurate measurements carried out with a D-GPS system, the system may be required remain stationary for an extended period of time, sometimes many hours, to obtain enough data to average to determine an accurate absolute location suitable for tamping operations. Such an approach is not practical.